As is known in the art, gas turbines employ rows of buckets or rotor blades on the wheels/rotor disks of a rotor assembly, which alternate with rows of stationary vanes on a stator or nozzle assembly. These alternating rows extend axially along the rotor and stator and allow combustion gasses to turn the rotor as the combustion gasses flow therethrough.
Axial/radial openings at the interface between rotating rotor blades and stationary nozzles can allow hot combustion gasses to exit the hot gas path and radially enter the intervening wheel space between bucket rows. To limit such incursion of hot gasses, the bucket structures typically employ axially-projecting angel wings, which cooperate with discourager members extending axially from an adjacent stator or nozzle. These angel wings and discourager members overlap but do not touch, and serve to restrict incursion of hot gasses into the wheel space.
In addition, cooling air or “purge air” is often introduced into the wheel space between bucket rows. This purge air serves to cool components and spaces within the wheel spaces and other regions radially inward from the rotor blades as well as providing a counter flow of cooling air to further restrict incursion of hot gasses into the wheel space. Angel wing seals therefore are further designed to restrict escape of purge air into the hot gas flow path.
Nevertheless, most gas turbines exhibit a significant amount of purge air escape into the hot gas flow path. For example, this purge air escape may be between 0.1% and 3.0% at the first and second stage wheel spaces. The consequent mixing of cooler purge air with the hot gas flow path results in large mixing losses, due not only to the differences in temperature but also to the differences in flow direction or swirl of the purge air and hot gasses.
In addition, the mixing of purge air and the hot gas flow results in a more chaotic flow of gasses across the platform of the turbine bucket. This increase in chaotic gas flow results in unequal heating of the platform during operation of the turbine, with attendant increases in thermal stresses to the platform and a resultant shortening of the working life of the turbine bucket.